1. Field of the Invention
This invention relates to an automatic attitude correcting system for use in an optical disc device for reproducing information from an optical disc.
2. Description of Related Art
In a reproducing operation of an optical disc on which information tracks are spirally or concentrically formed, if recording density of the optical disc is low, no problem would occur to the reproduction even when the control accuracy of control mechanisms containing an optical system including an optical head in its focus control, tracking control, tilt control, etc. is relatively low, and further no critical problem would occur even when an RF reproduction signal outputted from the optical head is deteriorated. The deterioration of the RF reproduction signal means deterioration in frequency characteristics, SN ratio, jitter, symmetry of waveforms, and the like.
If the recording density is high, the control accuracy of the control mechanisms is required to be considerably high, and the deterioration of the RF reproduction signal is required to be limited. However, actually, if the positional relationship between the optical disc and the optical head for reading the recorded information from the optical disc is not accurately kept, the RF reproduction signal outputted from the optical disc is degraded, so that high precision control cannot be performed.
For example, when tracking control is conducted, an objective lens of the optical head is shifted (displaced) in a radial direction of the optical disc (this shift is called "lens shift"), and during this lens shift, the optical axis of a laser beam emitted from a laser source toward a disc surface is not necessarily aligned with the center of the lens, and this error of alignment causes false signals to be generated and superposed on a focus error signal representing an out-of-focus amount of the objective lens, and on a tracking error signal representing the deviation of the laser beam from the center of a track, etc., so that no high-precision control can be performed.
Further, it is preferable that the optical axis of the laser beam is perpendicular to the surface of the optical disc. However, actually, when loading accuracy of the optical disc into an optical disc recording and/or reproducing apparatus is low or when the surface of the optical disc has a runout, the optical disc is inclined (no longer perpendicular) to the optical axis of the laser beam (hereinafter referred to as "tilt"), and thus false signals are generated and are liable to be superposed on the focus error signal, the tracking error signal, etc., so that the high-precision control cannot be performed, and the degradation of the RF reproduction signal increases.
Further, in order to reproduce an optical disc having high recording density, it is required to use an optical head with an optical system having a higher optical cut-off frequency. The use of such an optical head makes the requirement tighter for focus error tolerance, off-track tolerance, inclination (tilt) between the optical axis of a laser beam and an optical disc, etc. Accordingly, a focus error, an off-track amount and the tilt must be adequately limited. In this case, the most significant issue is that, in the control mechanisms, an operation point at which an error signal is minimum is not coincident with an operation point at which a reproduced RF signal (RF reproduction signal) becomes maximum. This is caused by occurrence of a DC (direct current) offset in a control error signal due to a mounting error of a control photodetector, deviation as aged, variation of temperature, etc.
In order to solve the above problems, automatic attitude corrections including positional correction are performed on the optical disc device before or during the reproduction of the optical disc. In this automatic attitude correction system, correction signals are first detected in the optical disc device to offset (cancel) the false signals, and this detected correction signals are supplied to the optical disc device at all times during the reproducing operation of the optical disc to offset the false signals.
That is, in the automatic attitude correction as described above, suitable correction signals are applied to the control mechanisms containing the optical system of the optical head for the focus control, the tracking control, the tilt control, etc., and the optimum values of these correction signals are determined or altered before or at a suitable time during the reproduction of the optical disc to maximize the amplitude of the RF reproduction signal outputted from the optical head.
Various kinds of automatic attitude correcting devices have been hitherto proposed for optical disc devices. As one type of these automatic attitude correcting devices, an automatic focus adjusting device has been proposed as disclosed in the Japanese Laid-open Patent Application No. 1-199326/1989. In this automatic focus adjusting device, a coil of a focus actuator for moving a lens is forcibly supplied with a correction signal current to move the lens. During the movement of the lens, the correction signal current is positively varied to measure the amplitude of the RF reproduction signal outputted from the optical head and set the value of the current so that the amplitude of the RF reproduction signal becomes maximum, and with this set value, the optical head is fixed.
Further, as another type of these automatic attitude correcting devices has been proposed as an automatic tilt correcting system for correcting an angle between an optical disc and an optical head as disclosed in the Japanese Laid-open Patent Application No. 1-199329/1989. In this automatic tilt correcting system, the amplitude of the RF reproduction signal is observed while positively varying the angle of the optical head to the surface of the optical disc, and the tilt angle of the optical head is controlled so that the amplitude becomes maximum. Further, in another type of automatic tilt correcting system as disclosed in the Japanese Laid-open Utility Model Application No. 4-45315/1992, the amount of jitter of the RF reproduction signal is observed while positively varying the angle of the optical head to the surface of the optical disc, and such a correction signal that the jitter amount becomes minimum is detected to correct the title angle of the optical head.
An optical disc involves generally various factors which disturb stability of the amplitude and jitter of RF reproduction signals, such as dust, scratches thereon, uneven thickness of a transparent substrate thereof, unevenness of refractive index of material, tilt (warp, runout) of the surface of an optical disc, etc. Therefore, in the conventional correcting methods for optical disc devices as described above, information recorded in an area having dust and scratches on the optical disc is reproduced with noises due to such dust and scratches, and thus when the RF reproduction signal for the information is utilized for attitude controls, accurate correction signals cannot be obtained.
In other words, in the automatic attitude correcting devices for the optical disc device, the values of correction signals which are calculated with reproduction signals reproduced from specific areas on the optical disc are not necessarily utilized as correction values for effectively correcting the other areas. That is, in some cases, these correction values are greatly deviated from optimum values (values of correction signals) in accordance with the rotational angle of the optical disc, and thus the amplitude of the RF reproduction signal may be greatly reduced.
In addition to the above problems, there is a problem that addition of a correction signal causes an increase of the maximum value of a residual control error.
On the other hand, in a method that an optimum correction signal value is detected in accordance with each of plural rotational angles on the optical disc and a correction signal is determined on the basis of the plural correction signal values thus detected, an operation of calculating the optimum correction signal value at every angle is required, and particularly when a correction value is calculated on the basis of an estimation calculation using a function approximation at every angle, a large load is imposed on a CPU, so that it becomes difficult to correct a correction value during a reproducing operation.